Method and apparatus for cold drawing



June 26, 1951 P. w. BRIDGMAN 2,558,035

METHOD AND APPARATUS FOR COLD DRAWING Filed July 5. 1947 2 SheetS-Sheet1 INVENTOR l3 1 Y Pliggv w. amosumu ATTORNEY June 26, 1951 Filed July 5.1947 P. W. BRIDGMAN IIETHOD AND APPARATUS FOR COLD DRAIING 2Sheets-Sheet 2 I an Y YI II'I'I rl l l J il hl l ll ll INVENTOR PERCY W.BRIDGMAN ATTORNEY Patented June 26, 1951 UNITED STATES PATENT OFFICEMETHOD AND APPARATUS FOR COLD DRAWING Percy W. Bridgman, Cambridge,Mass.

Application July 5, 1941, Serial No. 759,185

12 Claims. (01. 205-3) This invention relates to the technique ofextrusion and more particularly to a novel method and apparatus for colddrawing metals, alloys and other plastic materials.

It has been well established by experiment that many metals and otherplastic materials have greatly increased capacity for deformationwithout fracture if they are deformed while subjected to highhydrostatic pressure. For example, tension specimens of ordinary steelpulled while under a hydrostatic pressure of about 400,0000 p. s. i.elongated up to 300 fold (the reduction in area being 99.7%) beforefracture, whereas similar specimens pulled at atmospheric pressure brokeat elongations of less than two fold.

In view or the increased ductility imparted to metal by high pressure,it would appear that cold metal could be extruded through the die of aconventional extrusion press upon the application of suilicient pressuredirectly to the metal. However, such a procedure fails in practice,because the friction between the cold metal and the walls of the die isso great that the die will split before the pressure can be increasedsufficiently to start extrusion.

In my copending application, Serial No. 668,654, a method was describedwherein a solid plastic material such as a metal or alloy, is extrudedby subjecting such material uniformly to a high hydrostatic pressure,thereby increasing its ductility and forcing the material, while in anincreased state of ductility, through a die. In each of severalmodifications described therein, the body being extruded is subjected tohigh hydrostatic pressure on one side of the die and to atmosphericpressure on the other side. As a result, the body is under great axialcompressive stress during its passage through the die and tends todeform laterally against the inner walls thereof, thus creating greatfrictional forces in opposition to the forces promoting the extrusionprocess. It was disclosed therein that the hydrostatic pressure requiredto produce a given reduction of area could be considerably reduced bycombining the extrusion process with a drawing operation wherein apulling force is exerted on the issuing metal. This has thedisadvantage, however, that a large part of the material subjected totension is no longer under the influence of the hydrostatic forces thatconfer improved ductility. Consequently, brittle materials, ordinarilycapable of being extruded by hydrostatic pressure alone, frequently failby tension in the combined process.

It is the primary object of the present invention to provide a methodand apparatus for drawing a solid plastic material such as a metal oralloy through a die while under high hydrostatic pressure and in agreatly increased state of ductility. Another object of the invention isto provide a relief valve that is readily operable under highhydrostatic pressure. A still further object is to provide a simplemethod of firmly securing the reduced end portion of a body to beextruded to a guide member capable of exerting a high tensile force whenhydrostatic pressure is applied to the surrounding fluid.

In general, the method of the present invention comprises passing areduced portion of a body of solid plastic material through a die,subjecting the portions of said body on both sides of said die to highhydrostatic pressure and applying tension to said reduced portion ofsaid body, for example by causing said pressure to force an extendedportion of said reduced portion through an orifice.

The invention is further described with reference to the accompanyingdrawings, in which Fig. 1 is an elevation, in diagrammatic crosssection,of a hydrostatic drawing apparatus in accordance with the invention;

Fig. 2 is an enlarged perspective view, with a portion broken out, ofthe pull rod 28 of Fig. 1:

Fig. 3 is an elevation, in diagrammatic crosssection, of a modified formof the apparatus shown in Fig. 1;

Fig. 4 is a. sectional view taken along the line 4-4 of Fig. 3;

Fig. 5 is an elevation, in diagrammatic crosssection, of a preferredform of relief valve for use in the hydrostatic drawing apparatus of theinvention.

As shown in Fig. l, the apparatus of the invention comprises a hydrauliccylinder B having three chambers adapted to hold a liquid or othersuitable medium M under high pressure, and a relief valve l2. The upperchamber I3 is equipped with a plunger It for applying pressure to themedium M and is separated from the intermediate chamber l5 by thedrawing die It which is supported by a shoulder IT in the cylinder B.The intermediate chamber ii, in turn, is separated from the lowerchamber it by the extrusion plate l9, which has an orifice 20 and issupported on a shoulder 2|. The relief valve I! connected to the lowerend of the hydraulic cylinder B may be of any suitable design, but inview of the high pressures used is preferably constructed in the novelmanner described in connection with Fig. 5. In order to prevent leakagewith the high pressures required in the medium M it is desirable thatthe forward end 22 of the plunger I8 is slideably fitted to the mainportion thereof to provide space for compresslble packing rings 28 thatare forced outwardly against the cylinder walls under the infiuence ofendwise pressure. For the same reason, packing rings 28 should be usedat the seat of the threaded joint connecting the relief valve l2 to thecylinder B. The packing rings may be of lead or other suitable material.Lead packing rings have been found to be particularly effective inmaintaining pressures of 200,000 p. s. l. and higher.

It is not necessary that the medium M, employed in the drawingoperation, be a liquid. In fact, at the pressures employed, it is mostprobable that oil, for example, is congealed into a soft solid. It isnecessary, however, that a solid medium be notably softer than thematerial to be extruded and soft enough to transmit approximatelyhydrostatic pressure, i. c. it must support only a relatively smallshearing stress. An advantage obtained by using a soft solid rather thana true liquid is that possible leakage of the medium around the die andpiston may be more effectively prevented.

To assemble the apparatus for use, an end portlon 28 of the metal body26 to be drawn is reduced in cross-section, as by machining or grinding,over a sufficient length to allow it to be passed through the die l5 andinserted in the closely fitting axial hole 21 in one end of the pull rod28 which extends into the loosely fitting orifice 28 in the extrusionplate l8. With the novel design of the recessed end of the pull rod 28,the hydrostatic pressure exerted by the surrounding medium serves tojoin the pull rod securely to the reduced end portion of the bodyinserted in the recessed end thereof provided only that the edge of thejoint is sealed air-tight by a non-porous sealing material such as softsolder 29. Referring to Fig. 2, a portion of the wall thickness of theaxial hole 21 in the pull rod 28 is reduced to a few thousandths of aninch, for example by forming two diametrically opposite flattenedportions 38 that extend along the outer surface of the pull rod for alength substantially equal to the depth of the hole. When the edge ofthe joint is sealed the application of hydrostatic pressure on the outerportion of the pull rod causes the thick portions of the walls to beforced inwardly with great pressure against the rod, while the flattenedportions of the walls, being considerably thinner, readily give way bybuckling. By making the axial hole 21 deep enough, an extremely largegripping force may be obtained. The forward end of the pull rodpreferably is rounded to permit easy entrance thereof into passage 20 ofFig. 1.

With the apparatus assembled as shown in Fig. 1, pressure may be appliedto the medium M within the cylinder B by exerting a downward force uponthe plunger H. The equalization of pressure between the upper chamber l8and the intermediate chamber l8 takes place almost instantaneousLvthrough suitable passages 8| preferably disposed concentrically aboutthe orifice of the die I. By means of these passages the upper andintermediate chambers are so interconnected as to form a unitary drawingchamber subjecting both the entering and emerging portions of the bodyto a high hydrostatic pressure. The transmission of fluid pressure fromthe intermediate chamiber ii to the lower chamber l8,

however, is purposely retarded by providing only a minimum amount ofclearance between the pull rod 28 and the orifice 28. When the desiredhigh hydrostatic pressure has become equalized in all three chambers,the relief valve I2 is quickly opened, allowing medium M to escape fromthe lower chamber l8 at a much greater rate than it can enter throughthe clearance between the pull rod 28 and the orifice 28. The resultinggreat diiference in pressure between the intermediate chamber l5 and thelower chamber I8 drives the pull rod 28 through the orifice 28 andthereby simultaneously draws the metal body 26 through the die I8. Withproper coordination of the relief valve opening, the pull rod clearance,and the force applied to the plunger, the portions of the body enteringand leaving the die are subjected to high hydrostatic pressure and aretherefore in an improved state of ductility. Furthermore, under suchconditions of coordination, the drawing process may be continueduninterruptedly until the lower end of the pull rod 28 reaches the endof its travel, for example as shown by the broken line in Fig. 1.

The use of a pull rod 28 is not essential to the operation of theapparatus, as the mechanically reduced portion of the body itself may bemade of sufficient length to extend into the orifice 28, which need beonly slightly larger in diameter than the throat of the die It. In thiscase the extrusion plate I9 may be positioned quite close to the die It,allowing only enough space between these members for the unrestrictedflow of the oil or other medium from the passages 8| through the smallclearance space between the reduced portion 25 of the metal body and thewalls of the orifice 28.

It is sometimes diilicult to provide an amount of clearance between thepull rod 28 and the walls of the orifice 28 sufficient for easyinsertion of the pull rod, yet not so large as to cause an undesirableloss of medium from the upper chambers l3 and I5 under the highpressures used. This difliculty may be avoided by providing a minimum ofclearance between the extrusion plate orifice and the pull rod and byemploying means to keep the end of the pull rod away from the extrusionplate orifice until the pressure in all three chambers has beenequalized. A modification of the apparatus of Fig. l employing thesefeatures is shown in Fig. 3. In this embodiment, the hydraulic cylinderC has an upper chamber 8| separated from a lower chamber 82 by anextrusion bushing l8 supported at its flanged end 84 on the shoulder 45.The die 48 is coaxially mounted in the base of a cup-shaped die holder41 which has an outer cylindrical wall 48 slideably disposed within thecylinder C, and a coaxial inner cylindrical wall 49 which protrudesbeyond the outer cylindrical wall 48. Passages 58 extend from the spacebetween walls 48 and M to orifices lying within the confines of thediametrical dimension of an axially located recess 58 in the upperportion of the extrusion bushing 43. A collar 5| rests upon the shoulder52 in the inner wall of the cylinder C and at least portions of saidcollar flt slideablv around the protruding end of the inner wall 48. Asshown in Fig. 4, passages for the flow of medium past the collar areprovided by the spaces 58 between the slldeably fitting portions 88.However, if the entire inner surface of the collar is made to fitslideably around the protruding end of the inner wall, other passagemeans, such as holes extending through the thickness of the collaraseaoss s would be equally suitable. A liner ll adapted to screw intothe upper portion of the hydraulic cylinder ilrmly clamps the collar Iiagainst the shoulder 52. A compression spring 56 acting between thecollar Ii and the base ofthe die holder 41 is restrained from advancingthe latter toward the extrusion bushing 44 by shear pins S'I extendingfrom the slideably fitting portions ll of the collar I2 into theprotruding end of the inner wall ll. These shear pins extendfromholesinthecollar ll totheholesinthe inner wall It, the holes beingopposite each other when the spring 56 is sufficiently compressed toabut the edge of the outer wall 48 against the collar ii. The protrudingend of the inner wall 49 extends suillciently beyond the outer wall IIto enable the plunger II to establish contact therewith and shear theguide pins during the early part of its stroke. A relief valve is isconnected to the lower chamber I! as in the previous modification.

A portion of the body I! to be extruded reduced in cross-section, ashereinbefore de scribed, in order that it may be passed through the dieoriflce, but the reduced portion 82 is made of only such length that itor the end of an attached pull rod, is adjacent to but not within theorifice H of the extrusion bushing ll. with the relief valve it closed,suflicient force is applied to the plunger Bl to produce the desiredhydrostatic pressure in the oil or other medium M contained within thecylinder C. By means of passages 50 and GI this pressure immediatelyequalizes itself throughout the entire interior of the cylinder. Whilemaintaining the force on the plunger, the relief valve II is opened topermit some of the medium M to escape from the cylinder as the plungerdescends under constant application of force. In its descent, theplunger OI makes contact with the protruding edge of the inner cylinderwall 49 of the die holder 41' and severs the shear pins 51. This allowsthe compression spring 56 to force the reduced end I!ofthemetalbody6!,ortheendofapullrodat-' tached thereto, into the snuglyfitting oriflce I, and brings the die holder 41 to rest against ablocking means comprising the upper surface oi the flange ll of theextrusion bushing 43. The end portion 62 of the body within the oriilceI completely blocks the flow of oil from the upper portion of theapparatus into the lower chamber 42. A high hydrostatic pressure is thusmaintained in the upper chamber II and in the recess 58 whereas thepressure in the lower chamber 02 continues to drop through escape of oilthrough the open relief valve 58. When the pressure difference becomessufllciently great, the end portion 62 of the body is forced through theoriflee Si by hydrostatic extrusion. It is thus apparent that the bodybeing drawn is subjected to high hydraulic pressure both before enteringand upon emerging from the die, similarly as in the apparatus of Fig. 1.

With the high fluid pressures used. often ranging up to 250,000 p. s. i.and higher, ordinaw high pressure valves have been found unsuitable,particularly where rapid opening of the valve is of primary importance.Thus, in the apparatus of Fig. 1 it is essential that relief valve ll beadjusted from fully shut to fully open almost instantaneously, asotherwise a considerable amount of oil or other medium would passthrough the clearance between the pull rod and the walls of theextrusion orifice before the pres- Q suneintheiowerchamber would dropsufflciently fastfortherodtobcginextruding.

Apreferredformofreliefvalveshowninflg- 5 comps-hes a cylindrical housingH having an externallythreaded reduced portion ll adapted. for example.to cooperate with a threaded recess intheendofcylindersBorCinFigs. lor3. respectively.- An axial inlet passage 12 of small diameter extendsfrom the outer surface of the reduced portion II to the apex of aconical valve seat I3, forming the base of a recess 14 axially disposedin the end wall of the pressure cylinder II. An outlet passage Itextends from the side wall of the recess ll. in the vicinity of theconical seat II. to an external connection ll on the housingH.Withintherecess'llisaloosely fitting closure member such as a hardenedsteel ball ll adapted to eifect a fluid-tight seal of the inlet passageI! when the said ball is forced against the conical seat 13. Pressureagainst the ball is provided by contact with a boss II on the forwardend ll of a plunger acting within cylinder Ii. In view of the highpressures used this plunger preferably consists of two slideably flttedparts 8| and ll separated by a compressible packing 81 capable of beingdeformed outwardly against the cylinder walls under the influence ofendwise pressure. The mouth of the pressure cylinder II is sealed by thecylinder head II, which is seated against the shoulder 84. Fluid-tightseating of the cylinder head is assured by providing compressiblepacking 85 between the said cylinder head and the reduced portion I ofthe flrmly tightened gland nut 81. The cylinder head 83 has a long stem88 passing through an axial opening in the gland nut 81 and terminatingin 'a threaded end adapted for connectingtheaxialpassagelsinthestem 88with an auxiliary hydraulic pmnp.

In operating the relief valve for use in the drawing apparatus of theinvention. fluid medium F under pressure supplied by an auxiliary pumpdrives the plunger within the cylinder 15 against the hardened ball ll.causing the latter to be forced against the conical seat 13. Thiseffectively seals the inlet passage I2 and prevents the escape of fluidmedium from the cylinder of the drawing apparatus. Since thecrosssectional area of the relief valve plunger is very much greaterthan the effective area of the hardened ball. a relatively small fluidpressure from an auxiliary pump is sufllcient to seal the valve againstthe high pressure existing in the main cylinder of the drawingapparatus. To open the relief valve, pressure is removed from theplunger by releasing fluid medium F in any suitable manner. This permitsthe plunger and the hardened ball to descend under the influence offluid presure from the main cylinder. thus connecting passage I! withthe outlet passage II and thereby providing a path of escape for mediumfrom the main apparatus cylinder.

One advantage of the relief valve described is that it possesses easeand rapidity of operation. Furthermore. there is no tendency for any ofthe moving parts to stick; and the fact that the closure member isforced straight against the seat without rotation is of particularadvantage when using high pressures.

Metals and other materials. which cannot be successfully drawn at roomtemperature, may be hot drawn at much lower than normal temperamres bymeans of the method and apparatus of the invention. This modificationrequires only the application of heat to the specimen to be drawn.

The increased ductility of a material under high hydrostatic pressuremakes it possible in some instances to draw coiled rod through the die.Such rod straightens itself out in passing through the die and emergesin the same condition as if it had been straight initially. This featureis of considerable practical significance, since it permits an increasedlength of material to be drawn in a given size hydraulic chamber.

Due to the great increase in ductility, metals drawn under highhydrostatic pressure acquire a similarly great increase in strainhardening. The degree of strain hardening may be determined from theflow stress," which is an increasing linear function of the naturalstrain (natural logarithm of the ratio of initial area to final area).Because of the great strains induced by drawing under hydrostaticpressure, it is possible to strain harden to flow stresses much greaterthan the maximum which can be attained by drawing the metal throughreducing dies at atmospheric pressure without fracture.

While in this description specific examples of methods and means forcarrying out the invention have been described in detail, these examplesare given by way of illustration merely, and it is to be understood thatthe invention is not limited thereby but encompasses all methods andmeans within the scope oi the appended claims.

What is claimed is:

1. An apparatus for drawing a body of solid plastic material through adie while under the influence of hydraulic pressure, comprising a firsthydraulic chamber having at least one fixed end wall; a die coaxiallydisposed within said first chamber and having the emergent end thereoffacing said end wall; passage means connecting portions of said firstchamber adjacent said die; a coaxial passage in said end wall at leastequal to the diameter of the throat of said die; a second hydraulicchamber communicating with said coaxial passage; medium within bothchambers capable of transmitting hydraulic pressure; a relief valveconnected to said second chamber; and means for applying pressure tosaid medium.

2. An apparatus according to claim 1, comprising a spring for urgingsaid die toward said end wall; restraining means for said motionoperable by said means for applying pressure to said medium; andblocking means for maintaining a space between said die and said endwall.

3. An apparatus according to claim 2, comprising a die coaxially mountedin the base of a cupshaped member having an outer wall slidably disposedwithin said first hydraulic chamber, a

a coaxial inner wall having an end protruding beyond said outer wall,and passages interconnecting opposite faces of said base; a collaraffixed to the inner wall said hydraulic chamber; said collar havingportions slidably surrounding the protruding end of said inner wall ofthe cupshaped member and passages connecting portions of said cylinderon opposite sides of said collar; a compression spring between saidcollar and said base restrained by shear pins extending from said collarinto said inner wall of said cup-shaped member; and a plunger in saidhydraulic chamber capable oi establishing contact with and moving saidinner wall of the cup-shaped member to sever said shear pins.

4. An apparatus as claimed in claim 3, comprising a pull rod adapted tobe secured to a reduced end portion of a body capable of passing through8 said die, said pull rod having a larger diameter than the throat ofsaid die and adapted to flt snugly within the coaxial passage in saidend wall.

5. An apparatus according to claim 3, comprising a shoulder within saidhydraulic cylinder to act as a support for said collar. and a lineradap"- ed to fit within said cylinder, said liner having an externallythreaded portion adapted to cooperate with an internally threadedportion of said cylinder to permit clamping said collar rigidly inplace.

6. In an apparatus for drawing a body of solid plastic material througha die while the lateral surfaces of said body on each side of said dieare subjected to hydrostatic pressure, a pull rod having an axial recessin one end thereof adapted to receive a reduced end portion of saidbody, said recess having a reduced wall thickness at one or moreportions of the circumference capable of transmitting radial force tothe body portion within said recess when the wall of said recess issubjected externally to said hydrostatic pressure.

7. In an apparatus for drawing a body of solid plastic material througha die while the lateral suriace portions of said body on each side ofsaid die are subjected to hydrostatic pressure, a cylindrical pull rodhaving an axial recess in one end adapted to receive a diametricallyreduced end portion of said body, the wall thickness of said pull rodbeing reduced in one or more places over a length substantially equal tothe depth of said recess, whereby radial pressure is transmitted to thebody portion within said recess when the wall of said recess issubjected to external hydrostatic pressure, and a rounded portion at theother end.

8. An apparatus for passing a body of solid plastic material through adie while under the influence of hydrostatic pressure, comprising afirst hydraulic chamber having at least one fixed end wall; a diedisposed within said first chamber and having the emergent end thereoffacing said end wall; passage means connecting portions 01' said firstchamber adjacent said die; a coaxial passage in said end wall at leastequal to the diameter of the throat of said die; a second hydraulicchamber communicating with said coaxial passage; medium within bothchambers capable of transmitting hydraulic pressure; means for applyingpressure to said medium; and a relief valve comprising a housing havinga valve chamber communicating with said second hydraulic chamber, saidvalve chamber having a valve seat and a lateral outlet passage andopening into a pressure chamber; a closure member adapted to cooperatewith said valve seat to seal said inlet passage; a plunger comprisingtwo slidably fitted coaxial members of substantially equal diametersseparated by compressible packing material capable of being deformedoutwardly against the inner walls of said pressure chamber, said plungerhaving a coaxial boss capable of enterim said valve chamber and forcingsaid closure member against said seat; and fluid-tight passage meansassociated with said pressure chamber to actuate said plunger.

9. A method of drawing a body of solid plastic material through a die,which comprises reducing a portion of said body to a cross-sectionadapted to pass through the throat of said die, passing said reducedportion through the throat of said die. subjecting the lateral surfacesof said body extending on both sides of said die to a hllh hrassaoss 9drostatic pressure, and applying an axially directed force to said bodycapable of urging the unreduced portion of said body through said die.

10. A method of drawing a body of solid plastic material through a die,which comprises reducins a portion 01' said body to a cross-sectionadapted to pass through the throat of said die, passing said reducedportion through the throat of said die, and subjecting the lateralsurfaces oi said body extending on both sides of said die and atransverse surface adapted to transmit axial force to said body to ahigh hydrostatic pressure, said hydrostatic pressure acting against saidtransverse surface imparting a resultant axial force upon said bodycapable of urging the unreduced portion of said body through said die.

1'1. A method of drawing a body of solid plastic material through a die.which comprises reducing a portion of said body to a cross-sectionadapted to pass through the throat of said die, passing said reducedportion through the throat of said die. and subjecting the lateralsurfaces of said body on both sides of said die and a transverse surface01 the unreduced portion of said body to a high hydrostatic pressure,said hydrostatic pressure acting against said transverse surfaceimposing a resultant axial force upon said body capable of extruding theunreduced portion of said body through said die.

12. A method or drawing a body of solid plastic 10 material through adie, which comprises reducing a portion of said body to a cross-sectionadapted to pass through the throat of said die, passing said reducedportion through the throat of said die, affixing to said reduced portiona tension member of larger cross-section than the cross-section of saidreduced portion, thereby providing a transverse surface adapted totransmit axial force to said body, and subjecting the lateral surfacesof said body extending on both sides of said die, and said transversesurface to a high hydrostatic pressure, said hydrostatic. pressureacting against said transverse surface producing a resultant axial forceupon said body capable of pulling the unreduced portion of said bodythrough said die.

PERCY W. BRIDGMAN.

REFERENCES cr'rnn The following references are of record in the file ofthis patent:

Um'I'ED STATES PATENTS Number Name Date 524,504 Robertson Aug. 14, 1894524.505 Robertson Aug. 14, 1894 524,506 Robertson Aug. 14, 1894 524,508Robertson Aug. 14, 1894 1,767,038 Anderson June 24, 1930 1,777,611Grohek Oct. 7, 1930

